Geological Map of Pembrokeshire - Case Study Example

Name: Student Number: Essay Title: Subject: Table of Contents 1 Introduction 3 2 Tenby 5 3 Saundersfoot 13 4 Coastal Defense 16 5 Kilgetty Bypass 17 6 Field Techniques 20 7 Strike and dip 21 8 Conclusion 21 9 References 22 1 Introduction One of the country’s most captivating and eye-catching coastal scenery is found at Pembroke shire. The entire coastline of this coastal scenery has been included as a national park mostly because of its diversity, natural beauty and uniqueness. Saunders foot and Tenby are two historically rich and beautiful towns that are found at the heart of Pembroke shire. Geotechnical researchers have greatly benefitted from the uniqueness of these two villages. This has mainly been as a result of the unique natural slopes, the Carmarthen Bay super scenery and the architecturally captivating Kilgetty Bypass that lies beneath the concrete arched bridge. An almost unbroken historical sequence of the region’s geology is portrayed by the layers of rock that lie within its perimeters (Howells, 1999). Pembroke shire’s geological history dates back to over 600 million years ago in the pre-Cambrian era, with the formation of hard igneous rocks. The volcanic landscape at Pembroke shire was terribly flooded during the Cambrian era. As a result of the sedimentation along the coastline, a lot of sand stones were produced and these are the ones that we see today, exposed along the coast (Kelsall, 2003). I analyzed the variety of different rocks created over time while in Pembroke shire by a combination of tectonic plate movements and other environmental factors such as heat, weathering, pressure and flooding. There have been several earthquakes at Pembroke shire, the most recent one being experienced in 1892. This earthquake measured 5.1 on the Richter scale and caused minor damages on physical structures (David & Neil, 2007). Figure 1: Geological map of Pembrokeshire Source: Howells, 2010 The Ritec Fault is a major fracture in the earth’s crust that it runs along from Tenby to Pembroke Dock (BBC, 2004). The area is rich in a variety of rocks, especially sedimentary and igneous rocks. There was a clear presence of sandstone, mudstone, shale, limestone, basalt and rhyolite among other rocks (fig. 1). 2 Tenby In Welch, Tenby is known as Dinbych-y-Pysgod. It is translated as the little town of the fishes (BBC, 2000). This fishing village is renowed for it scenic and picturesque environment. The 4km stretch of sandy beaches surrounded by Castle Hill and the East Cliff make the area to stand out. The surrounding cliffs are made up of a variety of different historical rocks. Figure 2 shows a typical Tenby cliff, made up primarily of sedimentary rocks the cliff has a 50ͦ south facing dip (Fig 2.). The earth movements are the reason for the occurrence of the rock dips. This is the main reason why there are a lot of faults, folds and gravity. Fragments of volcanic ash are traced in the sedimentary rocks that are found at the coastline. This is possible when the rocks are put under the scrutiny of a microscope. Rocks in the area have buckled and crumpled into folds as a result of the compressional forces that originate from the tectonic plates (fig. 2). The cliffs are made up of a variety of different rocks since a clear prescence of sandstone, mudstone, limestone and shale can be seen. There are traces of iron and limonite that can be seen amongst the layers. This can be proved by the appearance of the yellowish-brown tints on the rocks. The discolouration of iron is likely to have occured as a result of iron sulphates reaction to acids present in the envirmoent (fig 2). Figure 2: Different types of rocks found on surrounding cliffs of Tenby beach. sedimentary rocks present. Table 1: Grain size of different rocks Source: adapted from Howells, 2010. Gravel sizes usually ranges from 2-64mm and they usually tend to heap up in large quantities as a result of waves and river actions (William, 2004). Sand stone on the other hand ranges from 125micrometres-2mm in size and are composed of calacium carbonate. It usually ranges from very coarse to very fine sand. Silt is usually coarser as compared to clay soil. There have been a lot of man-made interventions that have been aimed at creating beauty at Tenby beach. The sandy beaches at Tenby beach are as a result of the natural formations that occur in the area and the human efforts that help to prevent flooding in the area. There is a very powerful sea wall along the cliffs of Tenby’s beach. These sea wall is both ancient and modern because it has both old parts and new structures that have been added to it. This can be seen in figure 3a and 3b. Bricks manufactured using igneous rocks have been used to manufacture the old sea wall. The bricks are reinforced using a thick and strong layer of mortar. It is this combination of concrete and igneous rocks that makes the wall to be strong enough to withstand the pressure of the floods. The rocks contain iron sulphate and this can be ascertained by the fact that there is discoloration seen on the rocks. This discoloration comes about as a result of the reaction of water and the iron to form the iron sulphate. A small area on this wall was exposed by removing the outer oxidized rock with the help of a hammer. A drop of hydrochloric acid was then applied on the exposed surface. The experiment led to a fizzing reaction which proves that there is limestone in the rocks. Figure 3a: Old Masonry sea-wall. Figure 3b: Modern Concrete sea-wall with wave reflector Figure 3b is a picture that shows the entrance of the beach. The masonry walls are constructed using concrete to strengthen it. The modern seawall has a wave reflector which has the function of reflecting the energy of the wave from the wall back to the sea (fig. 3b). This is not only a flood defense observable mechanism (fig 4). The cliffs of the sea are protected from erosion by the strong reflector sea wall. Figure 4: basalt and rhyolite blocks laid down to prevent cliff bed erosion. This picture show blocks of hard basalt and rhyloilte rocks that lie on the shore and protect the cliff bed from being eroded. These blocks (basalt and rhyolite), just like all igneous rocks, are formed from the solidified magma/lava. These large rocks are bulky and sometimes many but clearly not water tight, this however is not a huge problem as this would be consumed by Tenby’s sandy beach (David & Neil, 2007). The rocks eventually become discolored as a result of oxidation, the deposition of minerals and weathering. All these are likely to lead to the formation of limestone. The weathering of all cliffs is inevitable. There are plenty of overhangs and fault lines due to physical, chemical and biological weathering this cliff’s face is a victim of rapid weathering (fig. 5). The main reason that chemical weathering occurs is rain. For example, carbonic acid in rain water may react with rocks such a limestone or hydration of rocks. Oxidation (reaction with oxygen) may occur and also cause deterioration of the rocks. Biological and physical weathering may be caused by the growth of roots for example. Also, the burrowing of animals and Tenby’s sub-zero temperatures inevitably adds further damage (David & Neil, 2007). The frost action caused by the cold winters experienced at Tenby results in crystal formation within the rocks. This leads to the slow but sure deterioration of the historic cliff. For health and safety reasons, retainers are used to support the physical structure of the cliff for safety and health purposes. The cliffs may wears away with time and break down piece by piece. The retainer is meant to prevent the falling down of loose rocks which would have otherwise been a threat to safety by being potential causes of injury. The quantity of loose rocks and fault lines are likely to increase with time. The rocks eventually become too many for the retainer to withstand their weight (fig. 5). Figure 5: Cliff showing clear signs of erosion, with some part of the cliff clearly broken away. Figure 6: Cliff with overhanging rocks and fenced bridge at the top. The cliffs bridge is surrounded by uneven rocks. Figure 6 shows that these overhanging rocks lack retainer support (fig. 6). These overhanging rocks are likely to give way to gravity with time. The face of the cliff has undergone considerable weathering and there is some visible lichen growth as seen in fig. 7. The products of weakened and weathered rocks are quickly removed by wave action. The weathering process has left the bedrock exposed as it has stripped away vegetation and regolith. . Figure 7: cliff with rough surface, lichen, and fault line and exposed bedrock The limonite and iron present in the rocks cause the orange colour to appear. The tectonic plate movements and weathering are the main reason for the formation of the faults that are seen in figure 7. The two agents sometimes cause the faults independently or they sometimes work together to form the faults. With the danger of dislodging rocks, it is important that a considerable amount of care is taken when on such field trips. The people on the field trip need to know how to calculate the distance they need to maintain away from the cliff that is considered safe for them. The method of calculating this distance is depicted in the equation shown on figure 8. B Figure 8: Formula of calculating the safe distance that should be maintained from the cliff. People visiting the site can calculate the minimum distance considered to be safe from the cliff by using the calculations and equations shown on figure 8. Assuming that the distance from the cliff is roughly 30 metres, the plump gun is used at an angle of 30 degrees as the highest point from the cliff. Using the following calculation, the minimal safe distance from a cliff may be calculated: Angle A=30 B =30 Height of observer = 1.77m Therefore 30 x 30tan + 1.77 = 19.09 This means that people visiting such sites should keep at least 20 metres away from the walls to avoid the danger of having rocks falling on them. Figure 9: cliff used to calculate minimum safe distance from cliff. 3 Saundersfoot Saundersfoot is currently a pretty little seaside resort, it ranges from yatching, fishing port, and family beach holiday as well as hippy-surf haven. The resort is famous for its well built 1829 harbour that was mainly used to facilitate the exportation of coal (Alliance of Religions and Conservation, 2005). The Ladies Cave, which is one of the most famous textbook caves in the world, is found just south of Saundersfoot harbour. When the carboniferous period was coming to an end, the rocks were broken down into numerous faults as result of being crumpled up into folds. This led to the creation of the photogenic cave that is seen today. Different traces of plant fossils have been found in a variety of the rocks that are common in this area. They are said to have been deposited during the tropical swamps over 300 million years ago (Howells, 1999). Figure 10: Ladies Cave Anticline (arch) The ladies cave is said to have been made up of shale, sandstone and mudstone. These three elements combine to form what stands as the ladies cave today. There is visible discoloration of the inner rocks that are much older. This shows that there are traces of iron and limonite. The outer rocks of the cave are considered to be much younger than the inner rocks. That is why they have maintained their original colour. The rhythmic layering between the rocks has clearly visible fault lines. Many rocks in the cave are not supported. This is because the cave has an arching nature. The pressure exerted may therefore cause the fold to succumb to gravity. Most of the area around Sandersfoot and the surrounding cliffs are made up of rhyolite and basalt, which are igneous rocks. One cliff around the seaside is shown in figure 11. In a bid to prevent the dislodged and loose rocks from falling, there is a small fence that has been erected around the cliff to help retain these loose rhyolite and basalt. The fence suffices for now in retaining the rocks but it has been under criticism due to its weak nature. Most people suspect that it might give in to the pressure some day and collapse. Pedestrians should therefore take responsibility over their own safety and exercise restraint. Figure 11: Saundersfoot cliff, surrounded by small fence retainer. Loose dislodged rhyolite and basalt rocks and debris accumulated at the fence. Road at the top of cliff. The cliff also experiences a lot of external pressure from the road at the top of it. The dipping road contributes to the overall instability of the cliff. This also increases the rate at which the cliff deteriorates. It has been proposed that humans need to intervene to help in maintaining the safety of the area (John, 2011). If nothing is done, the rocks that hang on the cliff will continue to pose major security threats. Most of the cliffs at Saundersfoot have never had human intervention and those that have had some form of intervention only had it very many years back. This is the reason why there are some areas around Saundersfoot that are not considered safe for human access. 4 Coastal Defense The coastal defense at Saundersfoot is said to be more limited than that at Tenby. Tenby is at a lesser risk of flooding as compared to Saundersfoot. Most of the cliff beds at Saundersfoot are not protected because they lack physical barriers. They do not have coastal defense walls and rip rap boulders (Alliance of Religions and Conservation, 2005). The cliff bed is therefore highly exposed to agents of erosion and it is at a very high risk of landslides. Figure 12: Saundersfoot beach The magnitude of the waves at Saundersfoot is monitored using a wave rider buoy. The system uses a shoreline management plan that draws the shoreline and creates awareness among members of the public concerning the size of the wave. Sustainable options are being reviewed by authorities of the Pembroke shire coast. These options include the implementation of coastal defense programmes and other measures such as protection against tidal floods (Alliance of Religions and Conservation, 2005). 5 Kilgetty Bypass The Kilgetty bypass is found at the dual carriage way at A477. The bypass is the point where the end of the dual carriage way appears. This route from London officially comes to an abrupt end at the bypass. This is what the name Kilgetty implies. The dual carriageway was designed as a two lane road initially. It passes right through rock cuttings in the centre of the heavily wooded region (Rybář et.al , 2002). The area around Kilgetty bypass is known to have poor soils. Figure 13a : Design and profile of Kilgetty by-pass Figure 17: Source Rybář et.al , 2002 Figure13b: The Kilgetty by-pass under construction, 1975 Figure 13c: The Kilgetty by-pass, anchors installed to support bridge. In an attempt to control the oblique sliding, there has been the continuous installation of rock anchors. The excess water that used to accumulate has caused the authorities to drill drainage holes to allow this water to move freely (Rybar et al, 2004) (figure 14). These holes appear in the directions that are faced by the cliffs. This plan was meant to put an end to the damage experienced by cliffs. A box girder was used to construct the bridge. This box girder is made up of reinforced concrete and steel. The deck experienced some uncontrollable forces that lead to its collapse during its construction. This caused the fatal injury of four people who were involved. This incident led to the vigorous checking and testing of the bypass and bridge before it was opened to the public. This was to ensure that it was safe enough and that it would remain stable, skid resistant and durable for many years to come. Figure 14: The Kilgetty by-pass, and surrounding cliff with drainage holes. Motorists are faced with the constant risk of being victims of the falling rocks and debris from the cliff. Rock bolts have been placed at such sites to act as anchors to the areas of the cliff that have loose rocks. This was meant to reduce the risk of the rocks falling and to make the surrounding cliffs more stable. Figure 15: Cliff that lines Kilgetty bypass, complete with rock bolts to increase slopes stability. 6 Field Techniques Name of tool used Reason for use. Geological hammer A rock hammer was the easiest method in this context to determine how hard the rocks were. The aggregate value of the impact can also be measured using the hammer. The hammer can be used to identify the different types of rocks at the sites. Fresh rock can be obtained by using the hammer. Gloves and Goggles These are safety equipment to provide protection of the hands and eyes when dealing with rocks. Compass and Inclinometer Geological structures could be measured using the equipment. Bearing ratios could also be made. Angles of slope could also be measured using the inclinometer. Bottle containing acid When acid is mixed with limestone, there occurs vigorous fizzing. Acid was to be used as a test for limestone. Camera Photographs of the scenery could easily be taken by use of a camera. This would be used for analysis and discussions later. Video Recorder Every detail of the trip could be captured using the video camera. This would be later observed and discussed. Recorded events would be more reliable than those captured on pen and paper. 7 Strike and dip The imaginary line that is created by intersecting a fault is referred to as the strike. The two attributes of a dip are the direction and magnitude. The magnitude of the dip is usually the dimensions of the maximum angle that is formed between the horizontal line and the plane. The direction is simply the one that the plane is inclined downwards towards (Freeman, 1999). Figure: 16 Illustrating strike and dip of an inclined plane (e.g., that of a limb of a fold, or that of a fault). Strike is due north; dip magnitude is 45ᵒ; dip direction east. page 17 Procedures in field geology By Tom Freeman 8 Conclusion The coast of Pembrokeshire is one of the world’s most fascinating national parks and sceneries. Some of the most captivating landforms and geological features can be seen in this area. A trip to this area can definitely widen the visitor’s horizon in relation to geology and introduce them to one of the most attractive and eye-catching sites in the world. Tenby, Kilgetty and Saundersfoot provide on with the opportunity to experience the richness of history. A range of field techniques learnt in class can be applied in these areas as there are plenty of opportunities to do this. Rocks and types of erosions can also be studied in these areas as there are there in plenty. 9 References Alliance of Religions and Conservation 2005, Suandersfoot, Viewed 7 August 2011, http://cistercian-way.newport.ac.uk/place.asp?PlaceID=125 BBC, 2004, The geology of Milford. Viewed 7 August 2011, http://www.bbc.co.uk/wales/southwest/sites/nhob_walk/walk1.shtml David, A & Neil, W 2007, Wales, Lonely Planet, Wales. Freeman, T 1999, Procedures in field geology, Wiley-Blackwell. p. 17. Howells, S.,2010. Geological History of Pembrokeshire. Viewed 7 August 2011, http://www.pembrokeshire-online.co.uk/geolmap.htm John, D 2011, Folds, Faults and Fossils- Exploring Geology in Pembrokeshire, Llygad Gwalch Cyf, New York. Kelsall, D., 2003. The Pembrokeshire Coast Path, Cicerone Press Limited. p. 11-32. Rybář, J., Stemberk, J., Wagner, P., 2002. Landslides: proceedings of the First European Conference on Landslides. Prague. William, B 2004, Space Physics, Journal of Geophysical Research, vol. 109, pp. 89-98. Appendix (Source: http://geology.about.com/library/bl/blrockident_tables.htm) Identification of Igneous Rocks Grain Size Usual Color Other Composition Rock Type fine dark glassy appearance lava glass Obsidian fine light many small bubbles lava froth from sticky lava Pumice fine dark many large bubbles lava froth from fluid lava Scoria fine or mixed light contains quartz high-silica lava Felsite fine or mixed medium between felsite and basalt medium-silica lava Andesite fine or mixed dark has no quartz low-silica lava Basalt mixed any color large grains in fine-grained matrix large grains of feldspar, quartz, pyroxene or olivine Porphyry coarse light wide range of color and grain size feldspar and quartz with minor mica, amphibole or pyroxene Granite coarse light like granite but without quartz feldspar with minor mica, amphibole or pyroxene Syenite coarse medium to dark little or no quartz low-calcium plagioclase and dark minerals Diorite coarse medium to dark no quartz; may have olivine high-calcium plagioclase and dark minerals Gabbro coarse dark dense; always has olivine olivine with amphibole and/or pyroxene Peridotite coarse dark Dense mostly pyroxene with olivine and amphibole Pyroxenite coarse green Dense at least 90% olivine Dunite Identification of Sedimentary Rocks Hardness Grain Size Composition Other Rock Type hard coarse clean quartz white to brown Sandstone hard coarse quartz and feldspar usually very coarse Arkose hard or soft mixed mixed sediment with rock grains and clay gray or dark and "dirty" Wacke/ Graywacke hard or soft mixed mixed rocks and sediment round rocks in finer sediment matrix Conglomerate hard or soft mixed mixed rocks and sediment sharp pieces in finer sediment matrix Breccia hard fine very fine sand; no clay feels gritty on teeth Siltstone hard fine chalcedony no fizzing with acid Chert soft fine clay minerals foliated Shale soft fine Carbon black; burns with tarry smoke Coal soft fine calcite fizzes with acid Limestone soft coarse or fine dolomite no fizzing with acid unless powdered Dolomite rock soft coarse fossil shells mostly pieces Coquina very soft coarse halite salt taste Rock Salt very soft coarse gypsum white, tan or pink Rock Gypsum Read More